25.2 Representing Carbohydrate Stereochemistry: Fischer Projections

Fischer Projections and Carbohydrate Stereochemistry

Fischer projections give you a way to flatten the 3D arrangement of a carbohydrate onto a 2D page while preserving all the stereochemical information. Since carbohydrates often have multiple chiral centers, getting comfortable with Fischer projections is essential for identifying sugar types, assigning D/L configuration, and recognizing relationships between isomers.

Fischer Projections of Carbohydrates

A Fischer projection uses a simple cross pattern at each carbon. The convention encodes 3D geometry:

• Horizontal lines represent bonds coming out of the page, toward you.
• Vertical lines represent bonds going into the page, away from you.

Each intersection of a horizontal and vertical line represents a carbon atom (usually a chiral center). You don't typically draw the carbon itself.

Rules for drawing a carbohydrate Fischer projection:

1. Orient the carbon chain vertically.
2. Place the most oxidized carbon at the top. For aldoses, that's the aldehyde ($\text{CHO}$). For ketoses, it's the carbon closest to the ketone.
3. Place the least oxidized carbon at the bottom, which is usually a primary alcohol ($\text{CH}_2\text{OH}$).
4. Draw substituents ($\text{OH}$, $\text{H}$) on the horizontal lines to the left or right of each chiral carbon.

Common monosaccharides you'll see drawn this way include D-glucose (an aldohexose), D-galactose (an aldohexose), and D-fructose (a ketohexose).

Determining Stereochemistry from Fischer Projections

D vs. L assignment depends on a single chiral center: the one farthest from the carbonyl group. For an aldohexose like glucose, that's C-5.

• If the $\text{OH}$ on that carbon points to the right, the sugar is D.
• If the $\text{OH}$ on that carbon points to the left, the sugar is L.

This is worth emphasizing: D or L is determined only by that last chiral center, not by every $\text{OH}$ in the molecule. Nearly all naturally occurring sugars are D-sugars.

Don't confuse D/L with optical rotation. D-glucose happens to be dextrorotatory (+), but D-fructose is levorotatory (−). The D/L label comes from the Fischer projection, not from the sign of rotation.

Relationships between sugars based on their Fischer projections:

• Enantiomers have opposite configurations at every chiral center. D-glucose and L-glucose are enantiomers: every $\text{OH}$ that's on the right in one is on the left in the other.
• Diastereomers differ at some but not all chiral centers. D-glucose and D-galactose are diastereomers.
• Epimers are a special subset of diastereomers that differ at exactly one chiral center. D-glucose and D-galactose are C-4 epimers. D-glucose and D-mannose are C-2 epimers.

Optical Activity and Isomerism in Carbohydrates

Any molecule with at least one chiral center (and no internal plane of symmetry) can rotate plane-polarized light. Most monosaccharides have multiple chiral centers, so they're optically active.

The types of stereoisomeric relationships you need to recognize:

• Enantiomers: Mirror images; opposite configuration at all chiral centers. They rotate plane-polarized light by equal magnitude but opposite sign.
• Diastereomers: Stereoisomers that are not mirror images. They have different physical properties (melting point, solubility, optical rotation).
• Epimers: Diastereomers differing at only one chiral center. This term is used frequently in carbohydrate chemistry.
• Anomers: These arise when a monosaccharide cyclizes. The carbonyl carbon (C-1 in aldoses) becomes a new chiral center called the anomeric carbon. The two possible configurations are designated $\alpha$ and $\beta$.

Converting Between Carbohydrate Representations

Fischer projection → Haworth projection (for an aldohexose):

1. Identify the $\text{OH}$ on C-5. Its oxygen attacks the C-1 aldehyde to form a six-membered ring (pyranose).
2. Groups on the right in the Fischer projection point down in the Haworth projection.
3. Groups on the left in the Fischer projection point up in the Haworth projection.
4. The $\text{CH}_2\text{OH}$ group (C-6) projects up for D-sugars.
5. At C-1 (the anomeric carbon), the $\text{OH}$ points down for the $\alpha$ anomer and up for the $\beta$ anomer in D-sugars.

Haworth projection → Fischer projection:

1. Open the ring by breaking the bond between the ring oxygen and C-1, regenerating the open-chain aldehyde.
2. Orient the chain vertically with the aldehyde at the top.
3. Groups pointing down in the Haworth go on the right in the Fischer.
4. Groups pointing up in the Haworth go on the left in the Fischer.

Haworth → chair conformation: The Haworth is already a ring, so converting to a chair means placing each substituent in its correct axial or equatorial position. For $\beta$-D-glucopyranose, all bulky substituents ($\text{OH}$ groups and $\text{CH}_2\text{OH}$) are equatorial, which is one reason glucose is the most stable and abundant aldohexose.